1. Field of the Invention
The present invention relates to a substrate for information recording media and a manufacturing method thereof, and more specifically to a substrate for information recording media used in information recording apparatuses such as hard disk drives and a manufacturing method thereof.
2. Prior Art
In recent years, there has been remarkable progress in information technology, and development of various types of information recording apparatus for storing information has been carried out with vigor. Hard disk drives (hereinafter referred to as xe2x80x9cHDDsxe2x80x9d) form the mainstream of such information recording apparatuses.
In an HDD, recording and playback of information are carried out by means of a magnetic head flying over a data zone formed on a magnetic disk substrate. Driving methods used include a CSS (contact start/stop) method and a ramp load method.
In the CSS method, a CSS zone in which uniform minute undulations of height several tens of nm are formed is provided along the inner periphery or the outer periphery of the magnetic disk substrate. The magnetic head flies over the data zone of the magnetic disk substrate while the magnetic disk substrate is rotating, and slides over the CSS zone of the magnetic disk substrate when the magnetic disk substrate stops or starts up.
In the ramp load method, the magnetic head flies over the magnetic disk substrate while the magnetic disk substrate is rotating, and is stored in a predetermined storage position when the magnetic disk substrate stops.
In both the CSS method and the ramp load method, while the magnetic disk substrate is rotating, the magnetic head is thus raised up slightly from the magnetic disk substrate, and flies over the surface of the magnetic disk substrate with a gap (hereinafter referred to as the xe2x80x9cflying heightxe2x80x9d) of several tens of nm maintained between the magnetic head and the surface of the magnetic disk substrate.
In an HDD, it is necessary to prevent the magnetic head from contacting the magnetic disk substrate while flying over the magnetic disk substrate, thus preventing the magnetic head from being subjected to excessive resistance. To this purpose, art has been known from hitherto in which precision polishing is carried out using a polishing agent containing loose abrasive grains of mean grain diameter in a range of 0.3 xcexcm to 3.0 xcexcm, and then etching is carried out using silicofluoric acid, thus forming a large number of minute projections referred to collectively as xe2x80x9ctexturexe2x80x9d on the surfaces of the magnetic disk substrate (Japanese Laid-open Patent Publication (Kokai) No. 2000-132829).
According to this prior art, upon etching the surfaces of the magnetic disk substrate with silicofluoric acid, polishing marks (abrasive marks) formed by the polishing agent during the precision polishing remain behind as projections, and hence a texture comprised of a large number of minute projections is formed on the surfaces of the magnetic disk substrate. Moreover, silicofluoric acid gives a slower etching rate and weaker etching than hydrofluoric acid or an aqueous solution of hydrofluoric acid containing potassium fluoride, and hence the surface roughness can be controlled to high accuracy.
Moving on, as the amount of information stored has increased enormously in recent years, there have been calls for HDDs that are small but have a large storage capacity. It has thus become necessary to increase the recording density of the data zone, and hence there have been calls to reduce the flying height (for example to 10 nm or less) to cope with this increase in the recording density of the data zone.
However, if the flying height is reduced, then the air layer between the magnetic head and the magnetic disk becomes thinner, and hence the floating stability of the magnetic head drops.
To reduce the flying height while maintaining stability, it is necessary not only to completely remove minute particles of foreign matter attached to the magnetic disk substrate so that the magnetic head will not collide with such foreign matter, but also improve the quality of the texture, thus improving the floating stability of the magnetic head. It is thus necessary to reduce the projection height of the minute projections as much as possible but increase the density of the minute projections, and moreover suppress variation in the projection height.
However, according to the prior art described above, even though the precision polishing is carried out using a polishing agent containing loose abrasive grains of mean grain diameter in a range of 0.3 xcexcm to 3 xcexcm, because the etching rate given by silicofluoric acid is low, abnormal linear or point-like projections having a large projection height are formed through the etching carried out after the polishing. As a result, there is a problem that if a magnetic disk manufactured from the magnetic disk substrate is driven with a flying height of 10 nm or less as described above, then there will be a risk of the magnetic head colliding with the abnormal projections, resulting in so-called head crashes or thermal asperity.
Moreover, according to the prior art described above, the maximum permitted value of the surface roughness Ra is 2.5 nm, and hence there is a problem of not being able to completely deal with the issue of the floating stability of the magnetic head at low flying height.
With the foregoing in view, it is an object of the present invention to provide a substrate for information recording media and a manufacturing method thereof, which allow an information recording medium to be driven reliably and stably even when the flying height is made lower than conventionally to cope with increased recording density of the data zone.
It is envisaged that, in the technical field of information recording media, in the future there will be calls to further increase the data zone recording density. To realize such an increase in the data zone recording density, it will be necessary to further reduce the flying height as described above, and hence to further reduce the projection height of the minute projections formed on the substrate surfaces.
One might think that to reduce the height of the minute projections, the depth of etching from the substrate surfaces should be reduced. However, if the depth of etching is reduced, then most of the compressed layers (polishing marks) formed through the polishing will remain, i.e. it will not be possible to remove the compressed layers sufficiently. As a result, when the substrate for information recording media is exposed to a high temperature during chemical strengthening treatment, sputtering or the like carried out after the minute projections have been formed, relaxation expansion will be brought about by the heat, and hence the minute projections will grow. Abnormal projections having a large projection height will thus arise, and hence there will be a risk of a magnetic head colliding with these abnormal projections and thus thermal asperity or head crashes occurring.
To obtain a desired floating stability suitable for reducing the flying height, it is thus necessary to reduce the projection height of the minute projections, and also suppress variation in the projection height which can occur due to relaxation expansion as described above. Specifically, to avoid the formation of abnormal projections having a large projection height, it is thought to be necessary to make the etching depth as large as possible and thus sufficiently remove the compressed layers.
The present inventors carried out assiduous studies from such a perspective, and as a result discovered that by adding an alkaline compound to hydrofluoric acid to produce an acidic solution containing a fluoride salt, and carrying out surface treatment on a substrate with this acidic solution, the surface roughness Ra of the substrate surfaces can be reduced reliably and the projection height can be kept down and made uniform; moreover, even upon subsequently carrying out heat treatment such as chemical strengthening treatment, relaxation expansion is not brought about by the heat, and hence minute projections having a low projection height can be formed at high density.
The present invention was achieved based on this discovery. Specifically, a method of manufacturing a substrate for information recording media according to the present invention comprises the steps of carrying out precision polishing on a glass substrate using a polishing agent and then carrying out surface treatment, wherein the surface treatment is carried out using an etching liquid containing both hydrofluoric acid and a fluoride salt.
Moreover, as a result of experiments carried out by the present inventors, it was found that if the amount of the alkaline compound added to the hydrofluoric acid is low, then there is no longer any significant difference from the case that the etching is carried out using only hydrofluoric acid and hence abnormal projections are prone to form as described above, whereas if too much of the alkaline compound is added to the hydrofluoric acid, then the number of minute projections is reduced and hence it is not possible to form a texture of a desired high density. There is thus an optimum range for the amount of the alkaline compound added to the hydrofluoric acid.
The present inventors thus carried out further assiduous studies from this perspective, and as a result discovered that if an etching liquid is used in which the ratio X of the sum of the concentration per mol of hydrofluoric acid and the concentration per mol of hydrogen ions to the sum of the concentration per mol of hydrofluoric acid and the concentration per mol of fluoride ions has been adjusted to be in a range of 0.3 to 0.98, preferably 0.56 to 0.93, then there is hardly any change in the projection height of the minute projections, and in the maximum height Rp of the projection height from the mean value thereof (hereinafter referred to as the xe2x80x9cmaximum projection heightxe2x80x9d), between before and after the heat treatment described above. Moreover, it is possible to obtain a substrate for information recording media having not less than 150 minute projections of projection height not less than 2 nm per 100 xcexcm2 (10 xcexcmxc3x9710 xcexcm) and with a maximum projection height Rp of less than 3.6 nm even after the heat treatment. As a result, a desired floating stability can be obtained even at a low flying height.
In the method of manufacturing a substrate for information recording media according to the present invention, the etching liquid is thus preferably prepared such that the ratio X of the sum of the concentration per mol of the hydrofluoric acid and the concentration per mol of hydrogen ions to the sum of the concentration per mol of the hydrofluoric acid and the concentration per mol of fluoride ions is in a range of 0.3 to 0.98, more preferably 0.56 to 0.93.
According to the above manufacturing method, a desired substrate for information recording media having minute projections at a high density can be manufactured, and moreover even if heat treatment such as chemical strengthening treatment or sputtering is carried out, the maximum projection height of the minute projections after the heat treatment can be kept down to less than 3.6 nm. A substrate for information recording media having an excellent floating stability even at a low flying height can thus be obtained.
Moreover, the above fluoride salt is produced by replacing hydrogen ions in the hydrofluoric acid with cations in the alkaline compound. However, it is undesirable for these cations to be divalent cations, since in this case a precipitate will be formed with the hydrofluoric acid.
The etching liquid is thus preferably a solution in which hydrogen ions have been replaced with other monovalent cations.
Furthermore, the monovalent cations are preferably alkylammonium ions, which tend not to produce a precipitate even if reaction occurs with another chemical substance in the etching liquid. It is undesirable for the monovalent cations to be ammonium ions, since in this case a precipitate will be produced with a silicon compound that leaches out from the glass substrate.
Moreover, when surface treatment is carried out on the glass substrate using an acidic etching liquid as described above, some of the constituent components of the glass substrate will leach out into the acidic solution, and hence a thin, soft altered layer having a low mechanical strength will be formed on the substrate surfaces. This altered layer can be removed by carrying out surface treatment using an alkaline solution.
In the method of manufacturing a substrate for information recording media according to the present invention, it is thus preferable for the surface treatment to comprise carrying out first surface treatment using an etching liquid as described above, and then carrying out second surface treatment using an alkaline solution.
By carrying out such second surface treatment using an alkaline solution, preferably an alkaline solution having a pH of not less than 12, after carrying out the first surface treatment, the altered layer described above can be removed, and hence the surface layer of the glass substrate can be hardened.
Moreover, as a result of further assiduous studies, the present inventors discovered that by carrying out pre-treatment using an acidic solution, preferably of pH not more than 4, as a treatment liquid before carrying out the surface treatment, a yet larger number of minute projections can be formed on the surfaces of the glass substrate.
In the method of manufacturing a substrate for information recording media according to the present invention, pre-treatment is thus preferably carried out at least once using an acidic solution (preferably having a pH of not more than 4) before carrying out the surface treatment.
According to the above manufacturing method, a yet larger number of minute projections can be formed on the surfaces of the glass substrate. As a result, a substrate for information recording media having a yet further improved floating stability at low flying height can be manufactured, and startup of the information recording medium can be carried out yet more smoothly.
Moreover, because the pre-treatment is carried out using an acidic solution, as in the case of the surface treatment described above, an altered layer is formed on the surfaces of the glass substrate.
After carrying out first pre-treatment at least once using an acidic solution, it is thus preferable to carry out second pre-treatment using an alkaline solution, before carrying out the surface treatment.
Moreover, the polishing agent used in the precision polishing preferably contains loose abrasive grains having a mean grain diameter in a range of 0.01 xcexcm to 3 xcexcm.
According to the above manufacturing method, the depth of the compressed layers formed on the glass substrate surfaces through the precision polishing using the loose abrasive grains (hereinafter referred to as the xe2x80x9ccompression depthxe2x80x9d) can be made to be in a range of 2 nm to 15 nm, and as a result the desired texture can be obtained. If the compression depth is less than 2 nm, then the projection height of the minute projections will become too low, thus hampering the formation of the texture. On the other hand, if the compression depth exceeds 15 nm, then the compressed layers will no longer be dotted around but rather will become continuous lines following the abrasive marks formed by the abrasive grains, and hence ridge shapes will be formed through the etching instead of point-like minute projections, which is undesirable.
Furthermore, the present inventors carried out further assiduous studies with an aim of effectively obtaining a texture having a desired number of projections per unit area with a desired projection height, and as a result discovered that if the SiO2 content in the glass composition is excessively high, then it becomes difficult to form the minute projections, whereas if the Al2O3 content is excessively high, then the probability of abnormal projections being formed becomes high, resulting in there being a risk of head crashes and thermal asperity arising.
It is thus preferable for the glass substrate to have a composition in which there is a certain relationship between the SiO2 content and the Al2O3 content; Specifically, it is preferable for the difference between the SiO2 content and half of the Al2O3 content to be in a range of 53 to 66 mol %, more preferably 58 to 64 mol %. Moreover, the glass substrate used in the substrate for information recording media is preferably made from an aluminosilicate glass, which has excellent water resistance and mechanical strength.
Moreover, the glass substrate used in the substrate for information recording media is preferably made from an aluminosilicate glass, which has excellent water resistance and mechanical strength. Specifically, the glass substrate preferably has a composition in a range of 63 to 70 mol % of SiO2, 4 to 11 mol % of Al2O3, 5 to 11 mol % of Li2O, 6 to 14 mol % of Na2O, 0 to 2 mol % of K2O, 0 to 5 mol % of TiO2, 0 to 2.5 mol % of ZrO2, and 0 to 6 mol % of MgO, 1 to 9 mol % of CaO, 0 to 3 mol % of SrO and 0 to 2 mol % of BaO, with an additional condition that the total content of MgO, CaO, SrO and BaO is in a range of 2 to 15 mol %.
Moreover, a substrate for information recording media according to the present invention, in which a large number of minute projections are formed on at least one surface of a glass substrate, is manufactured using a manufacturing method as described above. The substrate for information recording media is also preferably such that the density of minute projections having a projection height of not less than 2 nm is not less than 150 per 100 xcexcm2, and the maximum projection height from the mean value of the projection height is less than 3.6 nm.
According to the above substrate for information recording media, because the substrate for information recording media is manufactured using a manufacturing method as described above, and because the density of minute projections having a projection height of not less than 2 nm is not less than 150 per 100 xcexcm2, and the maximum projection height is less than 3.6 nm, it is possible to maintain a good floating stability such that head crashes, thermal asperity and the like do not occur even if driving is carried out at a low flying height of 10 nm or less.
To attain the object described above, the present invention provides . . .
(Claims 1 to 3) In a method of manufacturing a substrate for information recording media comprising the steps of carrying out precision polishing on a glass substrate using a polishing agent and then carrying out surface treatment, the surface treatment is carried out using an etching liquid containing both hydrofluoric acid and a fluoride salt, and moreover the etching liquid is preferably prepared such that the ratio X of the sum of the concentration per mol of the hydrofluoric acid and the concentration per mol of hydrogen ions to the sum of the concentration per mol of the hydrofluoric acid and the concentration per mol of fluoride ions is in a range of 0.3 to 0.98, more preferably 0.56 to 0.93. As a result, a desired substrate for information recording media having minute projections formed at a high density (texture) can be manufactured, and even if heat treatment such as chemical strengthening treatment or sputtering is carried out, the maximum projection height after the heat treatment can be kept down. A substrate for information recording media having an excellent floating stability even at a low flying height can thus be obtained.
(Claims 4 and 5) Moreover, the etching liquid is preferably a solution in which hydrogen ions have been replaced with other monovalent cations, and these monovalent cations are preferably alkylammonium ions. As a result, a precipitate will not be formed in the etching liquid, and hence the desired etching can be carried out smoothly.
(Claim 6) Moreover, the surface treatment preferably comprises carrying out first surface treatment using the etching liquid, and then carrying out second surface treatment using an alkaline solution. As a result, a soft altered layer having a low mechanical strength that is formed on the substrate surfaces through the first surface treatment can easily be removed through the second surface treatment, and hence the chemical durability of the surface layer can be increased.
(Claims 7 to 9) Moreover, before carrying out the surface treatment, it is preferable either for pre-treatment to be carried out at least once using an acidic solution, or for first pre-treatment to be carried out at least once using an acidic solution and then second pre-treatment to be carried out using an alkaline solution; in either case, the pH of the acidic solution is preferably not more than 4. As a result, a yet larger number of minute projections can be formed on the surfaces of the glass substrate, and hence smooth startup of a magnetic disk manufactured from the substrate for information recording media can be carried out yet more reliably, and the floating stability at low flying height can be yet further improved.
(Claims 10 to 11) Moreover, the composition of the glass substrate is preferably such that the difference between the SiO2 content and half of the Al2O3 content is in a range of 53 to 66 mol %, more preferably 58 to 64 mol %. As a result, suitable minute projections, specifically such that the density of minute projections having a projection height of not less than 2 nm is not less than 150 per 100 xcexcm2, and the maximum projection height is less than 3.6 nm, can be formed effectively.
(Claim 12) In addition, the glass substrate preferably has a composition in a range of 63 to 70 mol % of SiO2, 4 to 11 mol % of Al2O3 , 5 to 11 mol % of Li2O, 6 to 14 mol % of Na2O, 0 to 2 mol % of K2O, 0 to 5 mol % of TiO2, 0 to 2.5 mol % of ZrO2, and 0 to 6 mol % of MgO, 1 to 9 mol % of CaO, 0 to 3 mol % of SrO and 0 to 2 mol % of BaO, with an additional condition that the total content of MgO, CaO, SrO and BaO is in a range of 2 to 15 mol %. As a result, suitable minute projections, specifically such that the density of minute projections having a projection height of not less than 2 nm is not less than 150 per 100 xcexcm2, and the maximum projection height is less than 3.6 nm, can be formed effectively.
(Claim 13) Moreover, the polishing agent preferably contains loose abrasive grains having a mean grain diameter in a range of 0.01 xcexcm to 3 xcexcm. As a result, the compression depth of the compressed layers formed on the glass substrate surfaces can be made to be in a range of 2 nm to 15 nm, and hence the desired texture can be obtained.
(Claims 14 and 15) Moreover, a substrate for information recording media according to the present invention is manufactured using a manufacturing method as described above, and furthermore is preferably made to be such that the density of minute projections having a projection height of not less than 2 nm is not less than 150 per 100 xcexcm2, and the maximum projection height is less than 3.6 nm. As a result, a substrate for information recording media can be obtained that enables good floating stability to be secured even if driving is carried out at a low flying height of 10 nm or less.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.